Engineered fibers for well treatments

ABSTRACT

Mixtures of fibers and solid particles are effective for curing fluid losses and lost circulation in a subterranean well. Stiff fibers are more effective than flexible ones; however, mixtures of stiff and flexible fibers have a synergistic effect. The quantity and particle-size distribution of the solids are optimized according to the stiffness, dimensions and concentrations of fibers.

BACKGROUND OF THE INVENTION

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

In many well treatments it is necessary to inject a fluid into the wellunder pressure. If some or all of the fluid leaks out of the wellbore,this is termed “fluid loss”. If the treatment is one, such as drilling,in which the fluid is supposed to be returned to the surface, if some orall of the fluid does not return due to fluid loss, this is called “lostcirculation.” Lost circulation is a decades-old problem, but there isstill not a single solution that can cure all lost-circulationsituations. There are many available products and techniques, such aspolymer pills and cement plugs, to cure lost-circulation issues.

One of the simplest approaches is to add a lost circulation material(LCM) in the drilling fluid and/or in the cement or polymer system. LCMsystems often contain fibers. One of the major advantages of usingfibers is the ease with which they can be handled. There is a widevariety of fibers available to the oilfield. Most are made from naturalcelluloses, synthetic polymers, and ceramics, minerals or glass. All areavailable in various shapes, sizes, and flexibilities.

Fibers decrease the permeability of a loss zone by creating a porous webor mat that filters out solids in the fluid, forming a low-permeabilityfilter cake that can plug or bridge the loss zones. Typically, a veryprecise particle-size distribution must be used with a given fiber toachieve a suitable filter cake. Despite the wide variety of availablefibers, the success rate and the efficiency are not always satisfactory.

There is a need for fibers that are less sensitive to the sizes ofparticles in the fluids, that can block wide fractures even when theparticles are small, that can survive changes in pressure, that cancontrol spurt (the large amount of fluid often lost very quickly as thefilter cake is being formed), and that can control the total fluid loss.

Those skilled in the art will appreciate that the use of fibers in thecontext of lost circulation during well construction is distinctlydifferent from that associated with well-stimulation treatments such asacidizing and hydraulic fracturing. The principal differences betweenthe two applications are associated with permeability. The goal of lostcirculation control during drilling or primary cementing is to block theflow of wellbore fluids into the formation. This involves reducing thepermeability between the wellbore and the formation. On the other hand,the goal of stimulation treatments is to increase the effectivepermeability between the wellbore and the formation. Thus, anystimulation treatments involving fibers should not result in apermeability decrease.

Another distinction between well construction and well stimulation isthe fluid-flow direction. During drilling and primary cementing, fluidflow into the formation is generally to be avoided. The goal is todecrease the fluid-flow rate or stop it altogether. Conversely,stimulation operations are concerned with increasing the rate at whichfluids flow out of the formation and into the wellbore.

A notable application of fibers in the context of hydraulic fracturingis proppant flowback control. Fibers are mixed with proppant in a waysuch that, when the well produces, the fibers prevent migration ofproppant particles away from the fracture and into the wellbore. Yet theproppant pack containing fibers must remain permeable and allowefficient reservoir-fluid production. Such a condition would have noutility in the context of lost circulation control.

SUMMARY OF THE INVENTION

Methods are given for blocking fluid flow through one or more pathwaysin a subterranean formation penetrated by a wellbore. One aspectincludes selecting compositions, concentrations and dimensions of stifffibers and solid plugging particles, preparing a blocking fluidcontaining the stiff fibers and the blend of the solid pluggingparticles, and forcing the blocking fluid into the pathway. The pathwaytypically has one dimension at the formation face of at least about 1mm. The stiff fibers form a mesh across the pathway and the solidparticles plug the mesh and block fluid flow. The blocking fluidpreferably contains mica, and the solid plugging particles preferablyinclude calcium carbonate.

In one embodiment, the stiff fibers preferably have a Young's modulus offrom about 0.5 to about 100 GPa, more preferably from about 1 to about80 GPa. The stiff fibers preferably have a shortest cross-sectionaldistance of from about 80 to about 450 microns. The stiff fiberspreferably have a length of from about 5 to about 24 mm. The stifffibers have a value of the parameter S=Ed⁴/Wl³ in which S is thestiffness, E is Young's modulus, d is the diameter, W is the forcecausing a deflection, and 1 is the length, of from about 2 to about400,000 times the value of that parameter for a fiber having a modulusof 65 GPa, a diameter of 20 microns, and a length of 12 mm.

In the method, the stiff-fiber concentration is preferably from about2.85 kg/m³ to about 42.8 kg/m³. The fluid preferably also containsfibers that are selected from non-stiff fibers, differing stiff fibers,or both. The non-stiff fibers preferably have a Young's modulus of fromabout 0.5 to about 10 GPa, and a shortest cross-sectional distance offrom about 10 to about 100 microns. In this document, the terms“non-stiff” and “flexible” shall be used interchangeably. The totalfiber concentration is preferably between about 2.85 and 42.8 kg/m³.

In another embodiment, the concentration or composition of the stifffibers is varied during the treatment. In yet another embodiment, thepathway is a hydraulic fracture, and the fluid flow into the formationis blocked.

A further aspect of the invention relates to treating lost circulationby pumping the blocking fluid into the well continuously until fluidflow into the pathway is satisfactorily reduced. This procedure ismainly associated with drilling operations and primary cementing.

Yet another aspect of the invention relates to treating lost circulationby placing a discrete, desired amount of blocking fluid adjacent toand/or into the pathway. This procedure is primarily associated withremedial treatments such as setting plugs and squeeze operations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram depicting fiber deflection arising from anapplied force.

FIG. 2 shows the modified fluid loss cell used.

FIG. 3 illustrates one form of slot used.

FIG. 4 shows a comparison of fibers tested with 2-mm holes.

FIG. 5 shows a comparison of fibers tested with a 2-mm slot.

FIG. 6 shows results with fibers plus mica with a 2-mm slot.

FIG. 7 shows a comparison of fibers tested with a 3-mm slot.

FIG. 8 shows the effect of rate of pressure application a with 2-mmslot.

FIG. 9 compares results with one fiber and a grid with a 3-mm slot or agrid with three slots (3, 2, and 1 mm).

FIG. 10 shows a comparison of fibers with a 2-mm slot in a fluid oflower Solids Volume Fraction.

FIG. 11 shows a comparison of suitable and unsuitable fibers in a 2-mmslot.

FIG. 12 shows a comparison of suitable and unsuitable fibers in a 3-mmslot.

FIG. 13 shows a comparison of fibers using a combination of 1-mm, 2-mmand 3-mm slots.

FIG. 14 shows a comparison of fibers in an oil-based mud.

FIG. 15 shows a comparison of fibers with direct application of highpressure.

FIG. 16 shows fiber efficiency as a function of fiber concentration.

FIG. 17 shows fiber performance with varying solids volume fractions.

FIG. 18 shows results of including coarse particles at constant solidsvolume fraction.

FIG. 19 shows the effect of addition of mica flakes on the efficiency ofstiff fibers.

FIG. 20 shows fluid loss control as a function of varying theparticle-size distribution of the added particles in a water-basedfluid.

FIG. 21 shows fluid loss control as a function of varying theparticle-size distribution of the added particles in an oil-based fluid.

FIG. 22 shows the efficiency retained by stiff fibers at low solidsvolume fraction.

FIG. 23 shows the synergy between flexible and stiff fibers.

FIG. 24 shows the high-pressure performance of stiff fibers compared tobrittle or flexible fibers.

FIG. 25 is a diagram of a fluid-loss grid connected to an experimentalflow loop.

FIG. 26 is a diagram of a laboratory apparatus for simulating tangentialwall stress arising from fluid flow.

FIG. 27 presents the results of experiments evaluating the erosionresistance of filter cakes exposed to tangential wall stress.

FIG. 28 is a schematic diagram of an experimental flow loop that wasused in several examples.

FIG. 29 presents the effect of the ratio between stiff and flexiblefibers on spurt.

FIG. 30 presents the effect of fiber compositions on filter-cakestrength.

DETAILED DESCRIPTION OF THE INVENTION

Although the following discussion emphasizes blocking fracturesencountered during drilling, the fibers and methods of the invention mayalso be used during cementing and other operations in which fluid lossor lost circulation are encountered. The invention will be described interms of treatment of vertical wells, but is equally applicable to wellsof any orientation. The invention will be described forhydrocarbon-production wells, but it is to be understood that theinvention can be used for wells for the production of other fluids, suchas water or carbon dioxide, or, for example, for injection or storagewells. It should also be understood that throughout this specification,when a concentration or amount range is described as being useful, orsuitable, or the like, it is intended that any and every concentrationor amount within the range, including the end points, is to beconsidered as having been stated. Furthermore, each numerical valueshould be read once as modified by the term “about” (unless alreadyexpressly so modified) and then read again as not to be so modifiedunless otherwise stated in context. For example, “a range of from 1 to10” is to be read as indicating each and every possible number along thecontinuum between about 1 and about 10. In other words, when a certainrange is expressed, even if only a few specific data points areexplicitly identified or referred to within the range, or even when nodata points are referred to within the range, it is to be understoodthat the inventors appreciate and understand that any and all datapoints within the range are to be considered to have been specified, andthat the inventors have possession of the entire range and all pointswithin the range.

The inventors have surprisingly found that, in the use of mixtures offibers and solid particles to cure fluid losses and lost circulation, animportant factor in the selection and use of suitable fibers is thatthey must not be too flexible (bend too easily) or too brittle (breaktoo easily) for their length. In the present disclosure the term “stiff”will be used for suitable fibers. By stiff, it is to be understood thatthe fibers are neither too flexible nor too brittle.

The stiff fibers of the invention, suitable for curing even total lostcirculation situations in oilfield operations, must have a specificcombination of Young's modulus (as will be discussed in detail below),diameter or other cross-sectional dimension, and length. Fibersuitability may also be determined by chemical composition and state(for example crystallinity), dimensions and shapes (for examplecross-sectional shapes). Suitable stiffness is preferably a function ofYoung's modulus, length, and diameter (or longest cross-sectionaldimension if not circular)—these factors may compensate for one another.For example, a low-Young's modulus fiber may be “stiff” if it has asufficiently large diameter or is sufficiently short. Suitable fibersgenerally have a Young's modulus between about 0.5 and about 100 GPa,preferably from about 1.0 to about 80 GPa, more preferably from about1.0 to about 10 GPa and most preferably from about 1.5 to about 4 GPa.The fiber diameter (or, if not circular, the shortest cross-sectionaldimension) is generally between about 80 and about 450 microns,preferably about 100 to about 400 microns. The fiber length is generallybetween about 5 and about 24 mm, preferably about 6 to about 20 mm.

Suitable fibers have a “stiffness” to be more precisely defined below ofabout 100 to about 3000 times that of the glass fibers used in theexperiments described below, which typically have a 20-micron diameterand a Young's modulus of about 65 GPa. Fibers according to the presentinvention are used with a blend of plugging particles that may bealready present in the fluid or added to the fluid with the fibers. Theparticle-size distribution (PSD) of the blend of particles mayoptionally be optimized.

Although most of the experiments in the accompanying examples wereperformed with water-based fluids, the combination of suitable fibersand a particle blend may also be used in oil-base fluids. Optionally,wetting agents may be used to ensure that the materials are oil-wettablein oil-based muds or water-wettable in water-based muds. It will bewithin the general knowledge of the skilled person to perform laboratorytests to ensure fluid compatibility, that the fluid can transport theparticles at the required pumping rates, and suitability for the size ofthe openings in the fluid-loss pathways to be plugged. Fluids envisionedinclude, but are not limited to, drilling fluids, polymer pills, cementslurries, chemical washes and spacers.

The fibers may be used before or during operations such as cementing.Unlike fibers from the art, the stiff fibers and methods of theinvention are less sensitive to the particle sizes and fiberconcentrations in the fluids. In addition, they demonstrate betterresistance to pressure changes, and they provide robust performance interms of reproducibility, spurt control and fluid-loss control. Notably,unlike previous fibers in the art, they can cover wide fracturewidths—between about 1 mm to about 6 mm.

The fibers and methods of the invention provide solutions for a varietyof situations, including (but not limited to) curing lost circulation ofdownhole fluids, fluid loss during gravel packing, fluid loss duringwellbore consolidation treatments, cracking of cements, and otherproblems in oilfield operations. The invention may also be used inremedial treatments. One example may be the plugging of hydraulicfractures in formations that are no longer sufficiently productive.

Although many types, sizes, and shapes of fibers have been used in theart, the performance of these fibers depends mainly on the followingparameters: the solids content of the fluids (which generally has had tobe high), the fiber concentration (which generally has had to be high,especially to plug wide fractures), and a carefully selected and meteredparticle-size distribution. The stiff fibers and methods of theinvention make these parameters less critical. The fibers and methods ofthe invention may be used at lower fiber concentrations, and they can beused with less dependence on the solids content and the particle-sizedistribution of the fluid solids. However, careful attention to thesefactors results in less spurt and fluid loss than other LCM systems fromthe art.

The fibers and methods of the invention can be used to plug manyfracture widths, without the need to adjust the system variables, andwith a variety of pressure drops across the filter cakes formed. Theplugs easily sustain high pressure drops without failing. Without beingbound by any theory, it is believed that stiff fibers according to thepresent invention are not dependent on the optimized Packing VolumeFraction (PVF) concept. The PVF concept involves preparing fluids with amultimodal particle-size distribution. The amounts and sizes ofparticles are chosen such that the solids content in the fluid ismaximized, yet the fluid retains acceptable rheological properties.Optimized-PVF cement slurries are exemplified by CemCRETE™ technologies,available from Schlumberger.

For the current invention, the particles are optimized in a way suchthat they fit within the fiber network. Therefore, the optimalparticle-size distribution will not necessarily correspond to anoptimized-PVF system, and it may be designed to promote internalplugging by filter cakes either away from the wellbore or at thewellbore face.

Fluid losses are generally classified in four categories. Seepage lossesare characterized by losses of from about 0.16 to about 1.6 m³/hr (about1 to about 10 bbl/hr) of mud. They may be confused with cuttings removalat the surface. Seepage losses sometimes occur in the form of filtrationto a highly permeable formation. A conventional LCM, particularly sizedparticles, is usually sufficient to cure this problem. If formationdamage or stuck pipe is the primary concern, attempts must be made tocure losses before proceeding with drilling. Losses greater than seepagelosses, but less than about 32 m³/hr (about 200 bbl/hr), are defined aspartial losses. In almost all circumstances when losses of this type areencountered, regaining full circulation is required. Sized solids alonemay not cure the problem, and fibers are often needed. When losses arebetween about 32-48 m³/hr (200-300 bbl/hr), they are called severelosses, and conventional LCM systems may not be sufficient. Severelosses particularly occur in the presence of wide fracture widths. Aswith partial losses, regaining full circulation is required. Ifconventional treatments are unsuccessful, spotting of LCM or viscouspills may cure the problem. The fourth category is total losses, whenthe fluid loss exceeds about 32 m³/hr (about 300 bbl/hr). Total lossesmay occur when fluids are pumped past large caverns or vugs. In thiscase, fibers and sized solids alone might be ineffective, and the commonsolution is to employ cement plugs and/or polymer pills, to which fibersmay be added for improved performance. An important factor in practiceis the uncertainty of the distribution of zones of these types oflosses, for example, a certain size fracture may result in severe lossor total loss depending on the number of such fractures downhole.

Without wishing to be bound by any theory, the inventors believe thatthe mechanism of fibers in helping to form strong filter cakes is basedon three aspects:

-   -   1. Build/Bridge. The fiber should disperse well enough in the        fluid so that it can build or create a fiber mesh network        uniformly across the loss zone or zones, for example fracture        widths.    -   2. Plug. The fiber mesh should then be plugged with a blend of        solid particles to form a filter cake. The solids blend can        optionally be optimized, that is, designed according to the        porous structure created by the fibers, the porous structure        being a function of the fiber properties, such as aspect ratio        and elastic modulus.    -   3. Sustain. The filter cake of fiber mesh and solids should        withstand changes in pressure downhole. Changes in pressure can        occur, for example, due to pipe movements or changes in        hydrostatic pressure. Erosion may be caused by fluid circulation        in the annulus. Ideally, the filter cake should be able to        withstand the pressure changes and tangential erosion flow        downhole.

The inventors also believe that the solid particles in the fluid plugthe porous structure of the mat or mesh created by the contact pointswhere the fibers cross one another. The sizes of the openings betweenthe contact points are a function of the fiber diameter and aspectratio. Stiff fibers, having an aspect ratio in the range of about 10 toabout 300, can create a dense and homogeneous porous structure (mesh)with a large number of contact points, thus reducing the required sizeof the solid particles that are used to plug the porous structure. Thehomogeneous mesh formed by stiff fibers does not require the solidparticles to have a specific particle-size distribution to form a plug;therefore, the stiff fibers may be used with solids having a wide rangeof size distributions.

For non-stiff fibers, the porous structures are not well-defined, andexperimental results indicate that they do require more specificparticle-size distributions. Preferably, the particle size distributionof the solids may be chosen, or the particles already in the fluid canbe augmented, to take the porous nature of the fiber mesh into account.

The present inventive system employing stiff fibers and particle blendsis useful for curing fluid losses into fissures, natural fractures, andsmall vugs. The dispersed fibers, homogeneously dispersed orflocculated, dehydrating and coming together into a clump, reduce thepermeability of the loss zone or zones by creating a fibrous mesh. Fibershape, surface properties and stiffness help to determine the extent ofdispersion; for example, for a given fiber concentration and aspectratio, fibers with different shapes and stiffnesses will exhibitdifferent dispersion characteristics.

The fluid solids, including small cuttings if present, are trapped inthe pores of the fibrous net. Fibers with larger diameters will form amesh with larger openings (pores). The fiber flexibility can also havean influence. In addition, fibers having higher aspect ratios generallycreate a larger number of contact points per individual fiber.Furthermore, “non-stiff” fibers, for example typical multifilamentpolymer fibers such as those made of polypropylene, are flexible and canbend and overlap with neighboring fibers. This increases the particlediameter required to plug the openings between the contact points.However, stiff fibers (for example, the R1 polyvinyl alcohol fiberdescribed later) do not bend as easily and therefore require fewer or nocoarse particles.

The reduced dependence on the particle-size distribution of the solidsis an important feature of the invention. Nevertheless, the solidparticles preferably comprise a blend of coarse, medium, and fineparticles. The coarse particles in the blend preferably have an averageparticle size above about 180 microns and below about 1000 microns; mostpreferably they have a particle size of between about 700 and about 850microns. Particles having an average particle size of between about 30and about 180 microns, preferably about 150 and about 180 microns, forexample about 130 microns, can be used for the medium particles. Thefine particles are have sizes below about 30 microns, preferably havingan average particle size of from about 10 to about 20 microns. Theprincipal advantage of the fine particles is that they facilitatemetering and handling of the blend; alternatively, the fine particlescan be left out if the mixing and pumping equipment can handle blends ofmedium and coarse particles. If fine particles are required and caninvade small formation pores, non-damaging particles are preferablyused. The optimal ratio of the coarse/medium/fine particles varies,depending on the type of fiber.

The solid particles may be selected by one skilled in the art from oneor more members of the list comprising carbonate minerals, mica, rubber,polyethylene, polypropylene, polystyrene, poly(styrene-butadiene), flyash, silica, mica, alumina, glass, barite, ceramic, metals and metaloxides, starch and modified starch, hematite, ilmenite, ceramicmicrospheres, glass microspheres, magnesium oxide, graphite, gilsonite,cement, microcement, nut plug and sand. Carbonate minerals arepreferred, and calcium carbonate is most preferred. Mixtures ofdifferent types of particles may be used. It will also be appreciatedthat suitable particles are not limited to the list presented above.

Coarse, medium and fine calcium-carbonate particles may haveparticle-size distributions centered around about 10 microns, 65microns, 130 microns, 700 microns or 1000 microns, in a concentrationrange between about 5 weight percent to about 100 percent of theparticles. Mica flakes are particularly suitable components of theparticle blend. The mica may be used in any one, any two, or all threeof the coarse, medium, and fine size ranges described above, preferablyin a concentration range between about 2 weight percent to about 10weight percent of the total particle blend. Nut plug is preferably usedin the medium or fine size ranges, at a concentration between about 2weight percent to about 40 weight percent. Graphite or gilsonite may beused at concentrations ranging from about 2 weight percent to about 40weight percent. Lightweight materials such as polypropylene or hollow orporous ceramic beads may be used within a concentration range betweenabout 2 weight percent to about 50 weight percent. The size of sandparticles may vary between about 50 microns to about 1000 microns. Ifthe particles are included in a cement slurry, the slurry density willpreferably bet between about 1.0 to about 2.2 kg/L (about 8.5 to about18 lb/gal).

Many sizes and shapes of stiff fibers may be used. Stiff cylindricalfibers preferably have an aspect ratio between about 10 and about 300.Stiff rectangular fibers preferably have a thickness between about 20microns and about 100 microns, a width between about 100 microns andabout 450 microns, and a length between about 5 mm and about 24 mm. TheYoung's modulus of the stiff fibers according to the present inventionis important. In fact, it is desirable for the fiber to deform justenough under shear or restriction so that it will not break. On theother hand, in general, fibers with an excessively high Young's moduluscannot resist deformation without rupturing when pumped throughrestrictions. For example, glass fibers with a Young's modulus ofapproximately 65 GPa will break into pieces when pumped throughrestrictions during oilfield treatments.

It is known that sufficient solids concentrations and particularparticle sizes are necessary for the fibers to work. In the prior art,the pressure drop is essential to enhance the filtering process, and thefiber-plug performance may change with pressure drop. Thus, having ahigh solids concentration is one of the important criteria for fibers towork, but optimizing the particle size distribution and solids size iseven more critical.

As discussed earlier, the stiff fibers and methods of the invention donot necessarily need coarse particles to plug fractures, and theirstiffness plays a very important role in plugging severe fracturewidths. For example, stiff fibers can plug 3-mm fractures without coarseparticles. This is unique, as prior art fibers, for example glass andpolymer monofilament fibers, need 20 volume percent coarse particleswith 25 percent total fluid solids content to plug 2-mm fractures.Flexible fibers still cannot plug 3-mm fractures, even with furtherincreases in the coarse-particle concentration.

The use of the stiff fibers and methods of the invention minimizes thenecessity of using large-diameter particles to plug larger fracturewidths. Stiff fibers work effectively with suitable concentrations ofmedium particles or with a combination of medium and coarse particles.However, an optional optimized solids blend for a particular type ofstiff fiber can provide a solution to the uncertainty of fracture widthsand numbers downhole.

Unlike the prior art, the use of the stiff fibers and methods of thepresent invention extends the solids-concentration boundaries, and thesizes of solid particles that are sufficient to plug certain stiff fibermeshes. The different solid particles and additives present in drillingfluids and cement slurries typically have sizes in the range betweenabout 10 microns and about 1000 microns. The base fluid may be designedin such a way that it contains conventional LCM's, for examplemulti-modal sizes of particles of calcium carbonate or gilsonite,different sizes of mica flakes or nut plug, etc., and the solids contentof the fluid may range from about 10 percent to about 60 percent. Withthe use of stiff fibers the need for solids optimization is clearlyreduced. Stiff fibers are typically monofilaments. Flexible fibers aregenerally multifilaments for ease of handling, and are sold as tows.

Stiffness is proportional to the Young's modulus of a fiber, and isgenerally known as the resistance to deformation. Fiber stiffness is oneof the main characteristics affecting fiber performance. A simplifiedapproach to characterize fiber resistance is to consider the fiber to besimilar to structural beam, bending between two supports on each end.This is illustrated in FIG. 1, showing the deflection of a fiber oflength/, deforming under an applied load W.

Several assumptions were used to obtain an estimate of the fiberdeflection when exposed to a load. This was a simplified theoreticalapproach for estimating the strength of a fiber. The assumptions were asfollows:

-   -   Calculations were based on ambient conditions in air.    -   The load was the pressure drop acting directly towards the        fiber.    -   The load was uniform over the fiber length.    -   There was no fiber overlapping.        The load was calculated from the applied pressure (for example        70 gram-force/square millimeters (100 psi) and the fiber surface        area exposed to that pressure.        Fiber Deflection:

$\begin{matrix}{y = {\frac{5}{384}\frac{W\; l^{3}}{E\; I}}} & (1)\end{matrix}$Cylindrical Inertia:

$\begin{matrix}{{I_{c} = \frac{\pi\; r^{4}}{4}},{or}} & (2) \\{I_{c} = {0.0491d^{4}}} & (3)\end{matrix}$Rectangular Inertia:

$\begin{matrix}{I_{r} = \frac{t\; b^{3}}{12}} & (4)\end{matrix}$W=Weight or force causing the deflection (grams)E=Modulus of Elasticity (Kg/mm2)I=Moment of Inertia (mm⁴)l=Fracture width (mm)y=Deflection (mm)r=Fiber radius (micron)t=Fiber thickness (mm)b=Fiber width/breadth (mm)From the preceding equations, one may derive an expression forcalculating “stiffness.”

$\begin{matrix}{{S = \frac{E\; d^{4}}{W\; l^{3}}},{where}} & (5)\end{matrix}$S=stiffness.These equations may be applied to fibers of regular or irregularcross-sectional shape; as an example the calculation for fibers havingcircular cross sections is given below.

The deflection is proportional to 1/stiffness, and the W and l in Eq. 1were kept constant for all the fibers and the stiffness was thuscalculated. Table 1 presents “stiffness factors,” defined as the ratioof the stiffness of a given fiber to the stiffness of a glass fiber (GL)used in experiments that will be described later in the Examplessection. The glass fibers had a Young's modulus of 65 GPa, a 20-microndiameter and were 12 mm long. The nature of the polypropylene (FM),nylon (NL) and crosslinked-polyvinyl alcohol (R1 and R2) fibers willalso be described later in more detail. The calculation of the stiffnessor stiffness factor for the rectangular fiber is the same as for thecircular fibers, except that the inertia rectangle expression (Eq. 4)would be used.

TABLE 1 Stiffness Estimation Diameter/thickness E Fiber Material (um)(Kg/mm2) Stiffness factor 1. GL - 20 microns Alkaline resisted glass 206628.16 1.000 2. FM - 45 microns Polypropylene 45 152.96 0.591 3. NL -150 microns Nylon 150 203.94 97.356 4. NL - 250 microns Nylon 250 203.94751.202 5. NL - 280 microns Nylon 280 203.94 1182.031 6. FM - 12.5microns Polypropylene 12.5 152.96 0.004 7. NL - 50 microns Nylon 50203.94 1.202 8. R1 Crosslinked Polyvinyl alcohol 80 2957.18 1014.818 9.R2 Crosslinked Polyvinyl alcohol 100 2549.29 240.385It is believed that fibers with stiffness factors from about 2 to about400,000 are suitable, preferably between 4 to 12,000; and mostpreferably between 80 to 2,500. The stiffness comparison is not limitedto circular and rectangular fibers, but can be extended to fibers withother types of cross section.

Higher relative humidity and temperature adversely affect fiberstiffness. Stiff or, to some degree, thicker fibers help create a goodmechanical barrier or anchor in a fracture. In addition, particles orflexible fibers, preferably optimized in size and/or shape, effectivelyreduce the pore sizes between the stiff fibers. It is important to notethat a “stiff” fiber is not necessarily a hard or mechanically strongfiber. Suitable stiff fibers can have a Young's modulus of from about0.5 to about 100 GPa. Preferred stiff fibers have a Young's modulusbetween about 1.0 and 80 GPa, and most preferably from about 1.5 toabout 4 GPa. Such fibers (for their length and diameter) will beflexible enough to bend without breaking under oilfield conditions.Polymers such as polypropylene, nylon, and polyvinyl alcohol may fallwithin this range. In general, combination of low Young's modulus andlarger diameter, hence higher surface area than micron-diameter fibers,is highly preferred if the length is suitable.

Multifilament or bundled glass fibers of typical diameters are notsuitable, because of the high Young's modulus of glass, typically from50 to 90 GPa. Glass is brittle, not flexible, and cannot withstandhigher pressures across a fiber mesh. For example a pressuredifferential of 3.45 MPa (500 psi) will rupture the glass fibers.“Flexible mono-filament” fibers are not brittle, but are generally notstiff enough because their diameters are typically in the 10 to 80micron range. They are extremely flexible, and tend to deformexcessively and fail under high test pressures. On the other hand, itshould be borne in mind that bundles of fibers may have propertiesdifferent from individual fibers. For example, a bundle of severalmicron-sized flexible polypropylene fibers bonded together in a singlestrand may be a “stiff” fiber of the invention.

In a preferred embodiment, two (or more) different fibers may be used inthe invention. At present, they will be termed primary and secondaryfibers. The primary fibers must be stiff fibers, but may be of anycomposition that provides suitable properties. The secondary fibers maybe any fibers, stiff or not. When properly chosen, the primary andsecondary fibers may act synergistically. Stiff fibers preferably havelengths between about 5 mm and about 24 mm, most preferably from about 6to about 20 mm. Stiff fibers of the invention include (but are notlimited to) materials such as polypropylene, nylon, glass, Kevlar™, andcrosslinked polyvinyl alcohol. They are commercially available indifferent diameters and shapes. The specific gravity of stiff fibers ispreferably between about 0.90 and about 1.5, although denser materialsmay be used, for example certain metal ribbons like iron or aluminumalloys.

The secondary fiber may be an organic or synthetic type of fiber, forexample with a Young's modulus between about 0.5 GPa and about 100 GPa,preferably 0.5 GPa to 10 GPa. The fiber diameter may be between about 10microns and about 100 microns, preferably about 10 microns to about 50microns. The fiber length may be between about 5 mm and about 24 mm,preferably from about 6 mm to about 20 mm. Examples of secondary fibersinclude (but are not limited to) polypropylene, novoloid, Kevlar™ glass,nylon, polyamide, polylactic resin, polyvinyl alcohol, polyester, andcellulose.

The primary and secondary fibers may be of any fiber shape, for example,round, cylindrical, ribbon-like flat, coil-like spiral, trilobe, starshape, disoriented or irregular. Secondary fibers may also befibrillated. The secondary fibers may also be reactive fibers that canform a sticky fibrous net at certain temperatures, for example polyvinylalcohol or polylactic resin.

A suitable total fiber concentration is in the range between about 2.85and about 42.8 kg/m³ (about 1 and about 15 lbm/bbl), preferably fromabout 5.7 to about 22.8 kg/m³ (about 2 to about 8 lbm/bbl). The suitableratio of primary to secondary fibers is between about 95/5 and about30/70 by fiber volume, more preferably from about 90/10 to about 50/50.

The primary and secondary fibers do not necessarily need to have twodifferent chemical compositions. For example, nylon having a Young'smodulus of 4 GPa and a diameter of 150-400 microns can be the primary(stiff) fiber, and flexible multifilament nylon fibers with a diameterof 50 microns can be the secondary fibers.

There are many benefits of fiber blends. They are compatible with awider range of particles, in other words, they are less sensitive to thePSD of the plugging particles. They create a unique plug because theflexible fibers invade the fracture and thus anchor the plug, providingmuch better stability in terms of resisting erosion. Finally, theincorporation of flexible fibers (in this case, thin fibers) also helpsto suspend the thicker, stiff fibers that otherwise could not be usedalone because they would settle during injection.

The nature of the filter cake must also be considered. One of theproblems with filter cakes, even those including fibers, is that fluidcirculation can erode the surface of the filter cake by tangential flow.However, with a well-tuned system, stiff fibers can cause the plug toform internally, rather than at the entrance to the fracture, so thaterosion cannot happen. However, high fiber concentrations can poseoperational issues at the rig site, for example plugging of mixingequipment, or pump cavitation. The stiff-fiber concentration used in thefield should be between about 2.85 kg/m³ and about 28.5 kg/m³ (about 1.0lbm/bbl and about 10.0 lbm/bbl), without increasing the apparentviscosity of the fluid and compromising fluid pumpability.

To address lost-circulation effectively, it is highly preferred that thefibers follow the earlier-described three-step mechanism to build afilter cake. In fact, a failure in any part of this three-step processcan result in a plug failure. Fiber characteristics such as stiffnessplay a vital role in plug performance. Stiff fibers resist more pressurewith a smaller deflection, can build a structure corresponding to thefracture width, and trap optimized solids. Having stiff primary fiberscan also provide mechanical anchoring to other (secondary) fibers whenboth are used. It is equally important for the stiff fibers (for examplehaving diameters in the range of about 80 microns to about 450 microns)to have a sufficiently low Young's modulus that they can be pumpedthrough small restrictions while minimizing any breakage or blockageconcerns.

The inventors have established that, for a given type of fiber or fiberblend, increasing the fiber concentration improves the efficiency offluid-loss control. In addition to increasing the fiber concentration toachieve better fluid loss control, the Solid Volume Fraction (SVF) ofthe particles in the fluid may also be increased, i.e. adding more sizedsolids, to improve the fluid control efficiency. Alternatively,increasing the particle size of the added solids may also improve theoverall efficiency of fluid-loss control. Note that it would benecessary to ensure that the overall particle-size distribution of theadded particles was still in the suitable working range; otherwise,simply increasing the particle size would not result in increasedefficiency. Plate-like materials may also be used to better control thefluid loss.

The inventors have also established that, for a given fiber or fiberblend, the particle-size distribution (PSD) of the added particlesgoverns the permeability of the plugged fiber network. The selection ofthe proper PSD of the added particles is based on the pore-sizedistribution of the fiber network, and therefore depends on structuralparameters of the fibers. However, even if the PSD of the particles isproperly engineered, then an adequate concentration of particles in thefluid is still needed in order to achieve fluid-loss control. Aspreviously mentioned, the optimal PSD range depends on the structure ofthe fibers. For example, when thinner and more flexible fibers are used,addition of coarser particles is needed to control the fluid loss. Thereis a minimum solids volume fraction (SVF) in the fluid for the fibers tobe effective. For a given fiber system, any SVF above the minimum issuitable. The suitable SVF is between about 8 and about 50 percent, andis preferably between about 15 and about 35 percent. The fluidpumpability might become problematic if the SVF exceeds these limits.

The stiff fibers and solids are added to the drilling fluid (mud) in anyorder and with any suitable equipment to form the treatment fluid. Ifthe fluid already contains some or all of the solids necessary to form afilter cake on the mesh of stiff fibers, this is taken into account.Typically, the fluid containing the fibers and solids is mixed beforepumping downhole. The fibers can be added and mixed and then the solidsadded and mixed, or vice versa, or both fibers and solids can be addedbefore mixing. It may be determined that one of the components aids inthe suspension and/or dispersion of the other, in which case the helpfulcomponent is mixed into the fluid first. Typically, the treatment fluidis weighted to approximately the same density as the fluid previouslyinjected into the well. This practice minimizes migration of thetreatment fluid and helps prevent mixing with the previously injectedfluid. A weighting material may optionally be added to the fluid, thefibers, or the solids at any point. The treatment fluid can be added ina discrete amount, for example as a pill, or can be added continuouslyuntil lost circulation or fluid loss is satisfactorily reduced. Thetreatment fluid may be spotted adjacent to the location of the lostcirculation, if known, by methods known in the art.

The fluid containing the mixture of stiff fibers and solids may beinjected in several stages, in which the relative amounts of solids andfibers varies from stage to stage. Optionally, the stiffness of thestiff fibers may initially be less than optimal and then be increased toa suitable stiffness during the treatment. For example the concentrationof stiff fibers can be selected in the range of zero to alower-than-optimal concentration of stiff fiber in the first stage orstages of the treatment. A suitable low concentration can be determinedby measuring the minimal effective concentration of the stiff fibernecessary to form a mesh across a specific fracture size and then usinga concentration in the range of from about 10 to about 90 percent ofthat minimal effective blocking concentration. The selected lowconcentration should be tested in the same equipment to validate thenon-blocking effect of the treatment. The treatment with a lowconcentration of the stiff fiber is followed by a treatment with aneffective concentration of stiff fibers capable of rapid blockage.Effective concentrations can be determined by experiments describedlater. As a result, treatment with the effective concentration blocksthe fracture at or near the wellbore, and the low concentration stifffiber plugs the fracture at a bottleneck deeper in the fracture.

In another case, in addition to a change in the stiff fiberconcentration, the amount and/or size distribution of the plugging solidparticles may also be decreased. In general, whenever any changes aremade in the concentration or nature of the fibers, the concentration andparticle-size distribution of the plugging solids should bere-evaluated. The low-concentration treatment may be designed in such away that it blocks certain fracture sizes smaller than the originalfracture size. For example, the initial low-concentration treatment maybe designed to treat a 1-mm fracture, and the following treatment may bedesigned to treat a 4-mm fracture. For a 1-mm fracture, using zero to alow concentration of stiff fibers may be sufficient. When any of thesestrategies is followed, the treatment forms blockages at one or moredifferent depths in a fracture or in pores. One blockage may be close toor at the wellbore and another deeper in the fracture or pores.

Particularly, in the cases of severe or total losses, the stiff fibersand methods of the invention may be used as a pre-treatment before amore consolidated treatment. This use as a pre-treatment decreases thetotal cost, decreases damage to the formation, decreases furtherproblems that may otherwise appear because of delays in treatment, andincreases the chances of an effective first placement of the secondarytreatment (such as a cement plug or a reactive pill). Thus, the stifffibers and methods of the invention may be used in a first (primary)treatment for a temporary cure of severe or total losses. For greaterassurance of a permanent and complete treatment, it is convenient for adriller then to place a second treatment, such as a viscous pill or acement plug. In that case the composition and method assure that thesecond treatment is effective.

A suitable blend of stiff fibers and solids is selected for a givenfluid and given conditions, such as but not limited to the fluid type,the bottom hole temperature and the extent of losses being experienced.An effective concentration of each component, or a range of effectiveconcentrations, may be identified by performing experiments such asthose described below. Similarly, suitable compositions of solidparticles and stiff fibers, and suitable particle size ranges and fibersize and stiffness may be identified by performing experiments such asthose described below.

EXAMPLES

The present invention can be further understood from the followingexamples.

Experimental

Experiments were primarily performed with four different bentoniticwater-based muds. The density varied from 1.44 to 1.55 kg/L (12 to 12.9lbm/gal), and the solids content varied from 18 weight percent to 30weight percent. The water-based fluid formulations are shown in Tables 2to 5. PV is the plastic viscosity, and Ty is the dynamic shear stressfor Bingham fluids. Different formulations were prepared to observe theeffect of solids loading on the fiber performance. Calcium carbonate wasused as a lost-circulation material and barite as a weighting agent.

Most of the tests were performed in a modified lost circulation cell,shown in FIG. 2. The cell was equipped with modified slits through agrid, or a cylinder approximately 50 mm high having either a 1 mm to 3mm slot or 1 mm to 3 mm holes. FIG. 3 shows the arrangement with a slot.The experimental apparatus consisted essentially of a high-pressurehigh-temperature fluid loss cell 2 that is equipped with the cylinder 6at the bottom. Pressure was applied from the top of the cell onto fluid4 placed in the cell (as in traditional fluid-loss experiments). A valveat the bottom was closed, and a grid or cylinder having a slot or holeswas placed inside the cell. 300 mL of fiber-laden fluid was poured intothe test cell, and the cell was closed and pressurized to 0.69 MPa (100psi) to simulate the differential pressure at two ends of a fracture.Once the cell was pressurized, the bottom valve was opened quicklyenough to eliminate filtration of fibers through the bottom pipe. If thegrid was plugged, then the pressure was increased from 0.69 MPa (100psi) to 3.45 MPa (500 psi), in steps of 0.345 MPa (50 psi). The pressureincrease was purposely introduced to verify the strength of the filtercake. The pressure was held constant for at least 30 minutes, unless noplug formed or the plug failed. Mud loss monitored by collectingfiltrate in a container. The container was placed on a balance connectedto a computer, allowing one to record fluid loss over time.

During some experiments, the fibers were able to plug the slots at lowpressure; however, as soon as the pressure was increased, the plugfailed and the fluid inside the cell came out. If at any time the plugfailed, the test was stopped and the results were recorded.

A few tests were also performed applying 3.45 MPa (500 psi) pressureimmediately. These experiments were intended to evaluate the fibersystem's plugging behavior when suddenly exposed to higher downholepressures, for example surge and swab pressures, or to a loss zonesituation with an annular pressure of 3.45 MPa (500 psi) due to thehydrostatic column.

The bentonite particle size ranged from about 10 to about 100 microns.The “fine” calcium carbonate, hereinafter sometimes designated as the“10 micron” material, was about 10 percent smaller than about 1.1microns, about 50 percent smaller than about 8.6 microns, and about 90percent smaller than about 27 microns. The “medium” calcium carbonate,hereinafter sometimes designated as the “130 micron” material, was about10 percent smaller than about 15 microns, about 50 percent smaller thanabout 138 microns, and about 90 percent smaller than about 302 microns.The “coarse” calcium carbonate, hereinafter sometimes designated as the“1000 micron” material, was about 10 percent smaller than about 9.5microns, about 50 percent smaller than about 1026 microns, and about 90percent smaller than about 1500 microns.

TABLE 2 Bentonite Water-Based Mud Formulation 1 Component kg/m³ lbm/bblWater 815 285 Viscosifier Bentonite 51 18 Weighting Agent Barite 342 119Lost Circulation CaCO₃ - 130 m 171 60 Material 1 Lost CirculationCaCO₃ - 10 m 57 20 Material 2 Density: lb/gal [SG] 12.00 [1.43] SolidVolume Fraction 18 (SVF) (vol %) PV (Pa-s) 22 Ty [Pa (lb/100 ft²)] 8.6(18) 10 sec Gel 22 10 min Gel 34 API Fluid Loss 12 mL after 30 min

TABLE 3 Bentonite Water-Based Mud Formulation 2 Component kg/m³ lbm/bblWater 747 261 Viscosifier Bentonite 60 21 Weighting Agent Barite 200 70Lost Circulation CaCO₃ - 1000 m 130 45 Material 1 Lost CirculationCaCO₃ - 130 m 383 134 Material 2 Density: lb/gal [SG] 12.56 [1.50] SolidVolume Fraction 25 (SVF) (vol %) PV (Pa-s) 70 Ty [Pa (lb/100 ft²)] 13.4(28) 10 sec Gel 22 10 min Gel 45 API Fluid Loss 21 mL after 30 min

TABLE 4 Bentonite Water-Based Mud Formulation 3 Component kg/m³ lbm/bblWater 747 261 Viscosifier Bentonite 60 21 Weighting Agent Barite 350 122Lost Circulation CaCO₃ - 1000 m 60 21 Material 1 Lost CirculationCaCO₃ - 130 m 336 117 Material 2 Density: lb/gal [SG] 12.9 [1.54] SolidVolume Fraction 25 (SVF) (vol %) PV (Pa-s) 42 Ty [Pa (lb/100 ft²)] 15.3(32) 10 sec Gel 24 10 min Gel 52 API Fluid Loss 21 mL after 30 min

TABLE 5 Bentonite Water-Based Mud Formulation 4 Component  kg/m³ lbm/bbl Water 742 259 Viscosifier Bentonite 60 21 Weighting AgentBarite 198 69 Lost Circulation CaCO₃ - 130 m 510 259 Material 1 Density:lb/gal [SG] 12.56 [1.50] Solid Volume Fraction 25 (SVF) (vol %) PV(Pa-s) 36 Ty [Pa (lb/100 ft²)] 12.5 (26) 10 sec Gel 24 10 min Gel 48 APIFluid Loss 21.2 mL after 30 min

Seven fibers were used for most of the performance comparisons. Theseven fibers were two crosslinked polyvinyl alcohols (hereinafterdesignated R1 and R2), either of two multifilament polypropylene fibers(hereinafter designated FM), two nylon fibers (hereinafter designated asNL), and a glass fiber (hereinafter designated GL).

R1 was a non-water-soluble polyvinyl alcohol material in a flat shape(KURALON™ RF400 fiber available from Kuraray, Osaka, Japan). The lengthwas about 12 mm, the width was about 350 microns, and the thickness wasabout 80 microns. The melting point was greater than 200° C., and theYoung's modulus was 29 GPa.

R2 was also a non-water-soluble polyvinyl alcohol (KURALON™ RECS100fiber available from Kuraray, Osaka, Japan). The diameter was about 100microns. The melting point exceeded 200° C., and the Young's modulus was25 GPa. The fiber length was 12 mm.

The two FM fibers were also non-water-soluble. Both had a melting pointof 328° F. (164° C.). One fiber product was 12 mm long 45 microns indiameter (FIBERMESH™ 150-12, available from Propex, Inc., Chattanooga,Tenn., USA). The other fiber product was 19 mm long and 12.5 microns indiameter (SPECTER™ fiber, available from PGI Performance ConcreteFibers, Kingman, Kans., USA). Both were coated with less than 1 percenteach of a proprietary coating and a lubricant.

The GL fiber was about 12 mm in length and about 20 microns in diameter,and was obtained in tows of around 100 fibers packed together (CEMFIL™,supplied by Owens Corning, Toledo, Ohio, USA).

The NL fibers are available from DuPont, Wilmington, Del., USA under thetrade name TYNEX™. Another supplier is Rhodia Polyamide, Saint Fons, FR,under the tradename PA66™. The fibers have different densities;therefore, to make comparisons between equal volumes of fibers, thefiber concentrations were varied in such a way as to keep the fibervolume constant during the comparisons. The weight concentrationsrequired for equal volumes of some of the fibers are shown in Table 6.

TABLE 6 Number of Fibers Based on Fiber Volume Total Fiber vol. Based onFiber Single Fiber Fiber Conc. Conc. Total Number of Fiber Vol (mL)kg/m³(lbm/bbl) (mL) Fibers in 1.68 mL GL 3.77E−06 14.27 (5.00)  1.68445860 FM 2.36E−05 22.74 (1.79)  1.68 71338 R1 3.36E−04 7.47 (2.62) 1.685000 R2 9.42E−05 7.47 (2.62) 1.68 17834 NL 9.77E−04 6.15 (2.16) 1.681718

Example 1 Comparison of GL, FM, and R1+R2 with 2 mm Holes

A series of tests was performed with the apparatus described in FIG. 2,fitted with 2-mm holes. The following fiber systems were tested: GL, FMand a 50:50 mixture by volume of R1 and R2 fibers (hereinafterdesignated as R1+R2). The fibers were mixed into the fluid formulationof Table 2, but without the coarse (1000 micron) calcium carbonate. Tocompare equal volumes of fibers, the overall fiber volume was 1.68 mL in300 mL of base fluid. The results are presented in FIG. 4.

GL initially performed well. Low spurt was observed, and the filter cakeprovided good fluid loss control up to about 25 minutes. However, oncethe pressure rose to 2.41 MPa (350 psi), the filter cake failed. TheR1+R2 fiber system had a relatively high spurt—almost 100 mL. However,following the spurt, the filter cake provided good fluid-loss controlthroughout the 30-min test. The FM system performed well: low spurt andgood fluid-loss control throughout the 30-min test up to 3.45 MPa (500psi)

Example 2 Comparison of GL, FM, and R1+R2 with 2 mm Slot

The experiments of Example 1 were repeated, this time with the fluidsexposed to a 2-mm slot. Like Example 1, the overall fiber volume was1.68 mL in 300 mL of base fluid. The results are presented in FIG. 5.

The GL system demonstrated the lowest spurt. Excellent fluid-losscontrol was maintained until the pressure was raised from 0.69 MPa to3.45 MPa. The GL plug failed and all the fluid exited the apparatus.

The R1+R2 system began with 19 g spurt and showed only 26 g fluid lossafter 30 minutes. Unlike Example 1, the FM system was an instant failureat 0.69 MPa (100 psi). The total flow area for the 2 mm holes comparedto the 2 mm slot was 3:1. The spurt for R1+R2 and for the GL fibers wasalso approximately 3:1 between the 2 mm holes and the 2 mm slot. FM is ahighly flexible fiber, not a stiff fiber. It performed very well in the2 mm holes, but failed to build a fiber mesh in the 2 mm slot.

Example 3 Comparison of GL+Mica, and FM+Mica, with 2 mm Slot

This experiment compared the fluid-loss performance of a fluidcontaining both fibers and mica. The base fluid was the composition ofTable 5. The apparatus described in FIG. 2 was fitted with a 2-mm slot.The results are presented in FIG. 6

One 300-mL fluid contained 1.34 mL of GL fibers and 2.36 mL of mica.Notice that the amount of fiber was lower than the previous examples.This composition corresponds to 20 wt % fiber and 80 wt % mica, and ishereinafter designated as GL+mica.

The other 300-mL fluid contained 1.34 mL of FM fibers and 2.36 mL ofmica. Again, this composition corresponds to 20 wt % fiber and 80 wt %mica, and is hereinafter designated as FM+mica.

Both fiber/mica systems could plug the 2 mm slot, but the GL+mica-systemplug allowed a very high spurt, and then could not resist high pressureand lost its strength when the pressure changed from 1.72 MPa (250 psi)to 2.07 MPa (300 psi). The FM+mica system allowed much less spurt, andthe fluid-loss rate was slow thereafter.

Example 4 Plug Comparison of GL, FM, and R1+R2 with 3 mm Slot

A series of tests was performed with the apparatus described in FIG. 2,fitted with a 3-mm slot. The following fiber systems were tested: GL,FM, R1+R2 and R1 alone. The fibers were mixed into the fluid formulationof Table 4. To compare equal volumes of fibers, the overall fiber volumewas 1.68 mL in 300 mL of base fluid. The results are presented in FIG.7.

Neither the GL nor the FM systems could plug the slot at 0.69 MPa (100psi) pressure drop, but R1+R2 system plugged slot with a spurt of 15 gand a total fluid loss of 17 g after 35 min. R1 could plug the 3 mm slotalone, albeit with a larger spurt.

Not shown, is that either R1 or R2 alone could each plug the 2-mm slot,using the fluid formulation of Table 5.

The plug failures of GL systems at intermediate pressures are consistentwith the aforementioned theory of fiber stiffness. The stress on thefiber at the higher pressures exceeded the fiber's capacity, and thefiber could not sustain the higher pressures. An additional reasonbehind the GL failure may be the fiber's inability to build a fiber meshover the fracture width. A failure in any part of the process ofbuilding a mesh, plugging the mesh with particles, or sustaining theplug, results in failure.

Example 5 Plug Comparison GL, FM, and R1+R2, with Pressure-Drop Changes

Two series of tests were performed with the apparatus described in FIG.2, fitted with a 2-mm slot. The experiments tested filter-cake strengthversus pressure. One series involved subjecting the fiber-laden fluidsto a 3.45 MPa (500 psi) pressure-drop instantly. The other seriesinvolved incremental 0.69-MPa (100-psi) pressure increases to 3.45 MPa.The test duration was 35 min.

The following fiber systems were tested: GL, FM and R1+R2. The fiberswere mixed into the fluid formulation of Table 3. To compare equalvolumes of fibers, the overall fiber volume was 1.68 mL in 300 mL ofbase fluid. The results are presented in FIG. 8. In the figure labels,the “0.69-3.45” designation identifies systems that underwent theincremental pressure increases. The “3.45” designation means that thesystem underwent an instantaneous exposure to 3.45 MPa pressure drop.

When pressure was increased in 0.69 MPa increments, the GL systemsurvived all the way to 3.45 MPa. The recorded spurt was 4 g and thetotal fluid loss was 17 g. This result was better than a similarexperiment (presented in Example 2) involving a fluid without coarseparticles. However, when the pressure was raised directly to 3.45 MPa,the GL system failed instantly.

Similar behavior was observed with the FM system. It could plug the 2-mmslot and provide fluid-loss control when exposed to 0.69 MPa (100 psi)pressure drop increments up to 3.45 MPa. The spurt was 2 g and the totalfluid loss was 7 g after 35 min. However, when the pressure was raiseddirectly to 3.45 MPa, the FM system failed instantly.

The R1+R2 system could plug the 2-mm slot and provide fluid-loss controlwhen exposed to 0.69 MPa (100 psi) pressure drop increments. The spurtwas 10 g and the total fluid loss was 12 g after 35 min. However, unlikethe GL and FM systems, the R1+R2 system also provided fluid-loss controlwhen the pressure was raised directly to 3.45 MPa. The spurt was 5 g andthe total fluid loss was 8 g after 35 min. The success of the R1+R2system is attributable to the fibers' stiffness and ability to resistthe pressure.

Example 6 Plug Comparison of GL, FM, and R1+R2 with 3-2-1 mm Slot withImmediate 3.45 MPa (500 psi)

Tests were performed with the apparatus described in FIG. 2, fitted witha grid having 1-mm, 2-mm and 3-mm slots. The fiber laden fluids wereexposed to an immediate 3.45-MPa pressure drop. This experiment wasdesigned to show the performance of fibers when exposed to the realsituation of multiple fracture widths. The fluid was the formulationpresented in Table 3.

1.68 mL of GL, FM, or R1+R2 in 300 mL of base fluid could not plug the3-2-1-mm slot. The test was repeated with 2.68 mL of GL, FM, or R1+R2.Only the R1+R2 system was successful.

Another test was performed with 2.68 mL R1+R2 fiber; however, the testapparatus was fitted with a 3-mm slot only. As shown in FIG. 9, thefluid performed better without the multiple-slot-size grid.

One series was carried out with 2.68 mL of R1+R2 fiber. As shown in FIG.9, the system did not perform as well when tested against multiple slotsizes.

Example 7 Fiber Laden Fluid Effectiveness at Lower SVF

Most of the previous examples have involved fluids with a 25 percentSVF. This series involved the fluid formulation shown in Table 2, whichhas an SVF of 18 percent. During these tests the apparatus was fittedwith a 2-mm slot, 2-mm holes or a 3-mm slot.

The plugging behavior of R1+R2 fibers in a 25 percent SVF fluid has beenpreviously presented. For the reduced-SVF fluid tests, An R1+R2 systemwith a fiber concentration equivalent to 13.0 kg/m³ (5.0 lbm/bbl) GLcould plug both the 2 mm hole and the 2 mm slot. This concentrationcorresponded to 2.24 g of fiber in 300 mL of base fluid. The performanceof the R1+R2 system with the 2-mm slot is shown in FIG. 10. The spurtwas 119 g and the total fluid loss was 122 g after 35 min.

The same tests were performed with GL and FM systems. GL fibers wereadded at a concentration of 4.27 g/300 mL base fluid, and the FM fiberswere added at a concentration of 1.53 g/300 mL base fluid. Neithersystem could plug the 2-mm slot or 2-mm holes.

A similar comparison with the 18% SVF fluid was performed with fluidscontaining coarse mica. The results are presented in FIG. 10

1.34 mL of GL fibers and 2.36 mL of coarse mica were added to 300 mL ofbase fluid. The system failed instantly at 0.69 MPa when tested againstthe 2-mm slot.

1.34 mL of FM fibers and 2.36 mL of coarse mica were added to 300 mL ofbase fluid. This system also failed instantly at 0.69 MPa when testedagainst the 2-mm slot.

1.34 mL of R1+R2 fibers and 2.36 mL of coarse mica were added to 300 mLof base fluid. Unlike the others, this system performed well when testedagainst the 2-mm slot. The spurt was 12.5 g and the total fluid loss was13 g after 35 min.

For the distribution of particle sizes of the fluid of Table 2, theoptimal PVF is about 0.87. A fluid prepared with 0.87 PVF and 1.68 mL ofstiff R1 fibers could not plug a 3 mm slot. It failed instantly at 0.689MPa (100 psi).

However, the stiff R1 fibers performed exceptionally well in a fluidsystem with the same particle size distribution but a less than optimalPVF—0.84. The fluid composition is shown in Table 7. The density of thisfluid was 12.5 lbm/gal, and the SVF was 25%.

TABLE 7 R1 fluid formulation Ingredient kg/m³ (lbm/bbl) Water 742 (259)Bentonite 60 (21) Barite 198 (69)  CaCO₃ - 130 μm 510 (178)1.68 mL of R1 fibers in 300 mL of the base fluid of Table 7 plugged a3-mm slot

These results show that the choice of fibers can have a greater impactthan the solids composition. This was clearly the case with the 2-mmslot. The GL and FM fibers failed, but the stiff fibers of the inventionworked well. A fluid having a low solids content and no large (coarse)particles can plug fracture widths up to 2 mm with R1+R2 fibers, andincreases in fiber concentration or modifications of fluid formulationscan extend the performance window.

Example 8 Further Studies of Fiber Stiffness

Fiber stiffness is the key criterion governing the efficiency ofbridging the loss zone. Stiff fibers are more efficient in bridgingwider fractures. The following experiments demonstrate this effect.

The following comparison was made between 45-micron FM fibers and GLfibers. The apparatus of FIG. 1 was fitted with a 2-mm slot. Using thecomposition of Table 2, fluids were prepared with either 1.68 mL/300 mLFM or GL fibers at the same concentration. Tests were conducted at 0.69MPa (100 psi) differential pressure. As shown in FIG. 11, the moreflexible FM (polypropylene) fibers failed to provide fluid-loss control.All of the fluid was lost during the test. By contrast, the relativelystiffer GL fibers, under the same testing condition, provided fluid losscontrol.

The next experiment employed the 3-mm slot. 1.84 mL of GL fibers wereadded to 400 mL of the fluid of Table 3. As shown in FIG. 12, the glassfibers were not stiff enough to provide fluid loss control at 3.45 MPa(500 psi). On the other hand, stiffer nylon fibers (280-micron TYNEX™with a length of 16 mm), tested under the same conditions and with thesame fiber volume, provided excellent fluid-loss control and plugged the3-mm slot readily after losing less than 10 percent of the test fluid.This nylon fiber is more than 1,000 times stiffer than the glass fibers.

Using stiff fibers not only allows superior fluid-loss control forlarger fracture widths, but also improves the efficiency of controllingfluid loss over a wider range of fracture widths. This effect wasillustrated during the following experiments. The apparatus of FIG. 2was fitted with a grid having 1-mm, 2-mm and 3-mm slots. For eachexperiment, 2.68 mL of fibers were added to 400 mL of the fluid of Table3. Pressure was applied in 0.69-MPa (100-psi) increments up to 3.45 MPa(500 psi).

As shown in FIG. 12, the GL fibers lacked sufficient stiffness andfailed to plug the grid. However, the R1+R2 blend was successful. The R1and R2 fibers are approximately 1000 times and 240 times stiffer thanthe GL fibers, respectively. This feature minimizes the risk of notknowing the downhole-fracture widths and their distribution.

The same efficiency improvement obtained by using stiff fibers was alsoobserved when oil-base fluids were employed. The base fluid was a 1.44kg/L (12 lbm/gal) 80/20 OBM with the following composition.

-   -   492 kg/m³ EXXSOL™ D100 hydrocarbon fluid from ExxonMobil,        Irving, Tex., USA    -   13 kg/m³ VERSAMUL™ multipurpose emulsifier from M-I SWACO,        Houston, Tex., USA    -   13 kg/m³ VERSACOAT™ organic surfactant from M-I SWACO    -   9 kg/m³ VG SUPREME™ organophilic hectorite from Baroid        Industrial Products, Houston, Tex., USA    -   14 kg/m³ ECOTROL™ filtration control additive from M-I SWACO    -   23 kg/m³ Ca(OH)₂    -   45 kg/m³ CaCl₂    -   667 kg/m³ barite

This OBM also contained 280 lbm/bbl (797 kg/m³) of CaCO₃ particles. Thesize distribution of the CaCO₃ particles was 74:13:13 of particleshaving d₅₀s of 500 microns: 50 microns: 10 microns, respectively. Thefiber volumes were kept constant at 4 mL in 350.5 mL of the final fluid.As shown in FIG. 14, both the FM and GL fibers failed to control thefluid loss. The addition of stiffer fibers resulted in significantlyimproved plugging efficiency. In this case, the stiff fiber was a bundleof undispersed glass fibers (for example, CEMFIL™ 62/3 from OCV). Thereare approximately two to three hundred glass fibers in each bundle;therefore, the bundle behaves like a stiff fiber.

An important feature of stiff fibers is that they still exhibit theability to control fluid loss when the differential pressure isimmediately high (as opposed to increasing the pressure gradually). InFIG. 15, a comparison is shown for three different fibers: the flexiblepolypropylene fiber FM (having a diameter of about 45 microns), thebrittle glass fiber GL (having a diameter of about 20 micron) and thestiff R1 fiber. The fluid was the fluid of Table 4 and the fiberconcentrations were 1.68 mL in 300 mL. The slot width was 2 mm. Allthree formulations provided good fluid loss control at 0.69 MPa (100psi) of differential pressure. However, at 3.45 MPa (500 psi) pressure,the FM fiber system exited the slot as soon as the pressure was applied.The same result was observed for the GL system; in addition, the glassfibers were ruptured into smaller fragments during the experiment. Forthe stiff R1 fiber, the ability to control fluid loss was retained.

For a given type of fiber or fiber blend, increasing the fiberconcentration improves the efficiency of fluid-loss control. Theapparatus of FIG. 2 was fitted with the 3-mm slot, and the fluid ofTable 5 was used. The fiber was a blend of stiff, 280-micron nylon fiber(NL) and flexible 12.5-micron polypropylene (FM) fiber. The weight ratiowas 90% NL and 10% FM. As shown in FIG. 16, fluid-loss-controlefficiency was improved when the total fiber concentration was raisedfrom 2.35 mL (in 300 mL of final fluid) to 2.68 mL.

In addition to increasing the fiber concentration to achieve betterfluid loss control, the solid volume fraction (SVF) of the fluid mayalso be increased, i.e. adding more sized solids to improve thefluid-loss-control efficiency.

In FIG. 17, a comparison is made between two systems having blends ofstiff and flexible fibers (90 weight percent stiff nylon fibers and 10weight percent flexible propylene FM fibers), one with 20 percent SVFand the other with 25 percent SVF. The higher SVF fluid consistentlyexhibited better efficiency between 0.69 MPa (100 psi) and 3.45 MPa (500psi) of differential pressure. The tests were conducted with a 3-mmslot, and the overall fiber concentration was 2.89 mL in 300 mL of finalfluid. The base fluid in each case was the fluid shown in Table 5.

Alternatively, increasing the particle size of the added solids can alsoimprove the overall efficiency of fluid loss control. Note that, toobtain increased efficiency, it would be necessary to ensure that theoverall particle-size distribution of the added particles was still inthe suitable working range. The fluid of Table 5 was altered such that25% of the 130-micron CaCO₃ was replaced by 1000-micron CaCO₃. As shownin FIG. 18, adding the larger particles improved fluid-loss control.

Alternatively, plate-like material may also be used to improvefluid-loss control. Using 300 mL of the base fluid of Table 2, 2.36 mLof mica flakes were added with 1.34 mL of R1 fibers. The apparatus ofFIG. 2 was fitted with a 2-mm slot. As shown in FIG. 19, the fluid-losscontrol was better than a fluid with fibers alone. The same synergisticeffect has also been observed with flexible fibers and with a blend ofstiff and flexible fibers.

Example 9 Plugging Comparison Based on Fiber Structural Parameters andLost Circulation Material Particle Size Distribution

For a given fiber or fiber blend, the particle-size distribution (PSD)of the added particles governs the permeability of the plugged bridgingfiber network. The selection of the proper PSD is based on the pore-sizedistribution of the fiber network, and therefore depends on structuralparameters of the fibers. The use of stiff fibers facilitates bridgingof the open fractures; however, if the PSD of the added solids is notengineered properly, the bridged-fiber network will be too porous andfluid-loss control will suffer.

For example, the following five tests were performed under identicaltest conditions. The apparatus of FIG. 2 was fitted with a 3-mm slot,and the test pressure was 0.69 MPa (100 psi). For all tests, 1.68 mL ofthe R1+R2 fiber system were added to 400 mL of the base fluid describedin Table 5. The only variable was the PSD of the added solids. The PSDis denoted by the ratio of Fine:Medium:Coarse. The d₅₀ of the fineparticles was 10 microns; the d₅₀ of the medium particles was 180microns; and the d₅₀ of the coarse particles was 830 microns. Theresults are shown in FIG. 20.

Properly engineered ratios of the three categories of particlestypically provided improved efficiency. The 20:60:20 and 10:55:35 blendsprovided excellent fluid-loss control. In case more fluid loss isrequired, for example to build a thicker filter cake, then the PSD canbe tuned to increase the fluid loss. This is exemplified by theperformance of the 0:80:20 blend (FIG. 20).

The same trend of optimizing fluid loss control via the PSD was alsoobserved for an oil-base fluid. Just as in water-based fluids, toachieve optimal fluid-loss control, it is necessary to engineer andoptimize the PSD of the added particles. For these tests, the apparatusof FIG. 2 was fitted with the 3-mm slot. The base fluid has thefollowing composition.

-   -   492 kg/m³ EXXSOL™ D100 hydrocarbon fluid from ExxonMobil,        Irving, Tex., USA    -   13 kg/m³ VERSAMUL™ multipurpose emulsifier from M-I SWACO,        Houston, Tex., USA    -   13 kg/m³ VERSACOAT™ organic surfactant from M-I SWACO    -   9 kg/m³ VG SUPREME™ organophilic hectorite from Baroid        Industrial Products, Houston, Tex., USA    -   14 kg/m³ ECOTROL™ filtration control additive from M-I SWACO    -   23 kg/m³ Ca(OH)₂    -   45 kg/m³ CaCl₂    -   812 kg/m³ barite

The fiber blend was 63 weight percent of stiff 280-micron NL fiber(TYNEX™) and 37 weight percent of flexible 50-micron NL fiber (PA66™from Rhodia). The results are presented in FIG. 21.

The SVF was 25%; however, there were no fine particles. In an emulsion,the encapsulated droplets fulfill their function.

Even if the PSD of the particles is properly engineered, then anadequate concentration of particles in the fluid is still needed toachieve fluid-loss control. A comparison was made between three fibersystems. The base fluid of Table 2 was used, and 1.34 mL glass (GL),polypropylene (FM), of stiff R1 fiber were added 300 mL of base fluid.These tests were performed with the base fluid in Table 2, at 0.69 MPa(100 psi) in a 2 mm slot.

The PSD was 20 percent Fine/60 percent Medium/20 percent Coarse for theGL fiber and FM-45 micron fiber, and 10 percent Fine/90 percent Mediumfor the test with R1 fiber only. As shown in FIG. 22, the stiff R1fibers provided good fluid loss control, and the more flexiblepolypropylene and glass fibers failed. Reducing the criticality of thesolids content is another beneficial feature of using stiff fibers.

As previously mentioned, the optimal PSD range depends on the structureof the fibers and, to some degree, on the nature of the raw materials.For example, the thinner and more flexible fibers, like glass fibers andpolypropylene fibers, tend to favor a combination of fine, medium andcoarse particles, with the content of medium particles being the highest(see Table 8). For the thicker and stiffer fibers like nylon and R1fibers, it is not necessary to have the coarse particles. In otherwords, when thinner and more flexible fibers are used, addition ofcoarser particles is required to control the fluid loss.

TABLE 8 Optimal PSD range comparison Fiber Fine Medium Coarse Glassfiber 10-20 vol % 60-80 vol % 10-20 vol % Stiff Nylon fiber 10-20 vol %40-90 vol % 0% (280 microns in diameter) Flexible 10-20 vol % 60-80 vol% 10-20 vol % Polypropylene fiber (13 microns in diameter) R1 fiber10-20 vol % 40-90 vol % 0%

It was also found that substituting flexible fibers for some of thestiff fibers may also further improve the efficiency. For example, asshown in FIG. 23, comparing two systems with identical fiber volumes,when some of the stiff nylon fibers were replaced by flexiblepolypropylene fibers, the efficiency was greatly improved with a 3-mmslot. These tests were carried out with 1.68 mL of total fiber volume in300 mL of the base fluid of Table 5, which had a SVF of 25 percent. Forthe fiber blend, the weight ratio of the stiff 280-micron nylon to theflexible 12.5 micron FM was 90 to 10. This synergy was observed with awide range of fiber blends, ranging from 10 volume percent flexible/90volume percent stiff, to 90 volume percent flexible/10 volume percentstiff. It has been observed that, for these fibers and this PSD, themost robust ratio is 70 volume percent stiff fibers and 30 volumepercent flexible fibers.

Example 10 Stability of the Fiber Plugs Under Downhole Conditions

Increasing the differential pressure across the loss zone can render afiber plug ineffective. Tests were conducted with the fluid shown inTable 5, with a fiber concentration of 1.68 mL in 300 mL of the basefluid. The apparatus shown in FIG. 2 was fitted with the 2-mm slot.Results are presented in FIG. 24.

Flexible polypropylene fibers, FM, were shown to be ineffective evenunder the relatively low pressure of 0.69 MPa (100 psi). Brittle glassfibers, GL, are stiffer than polypropylene; therefore, they providedfluid-loss control for the 2-mm slot, but they failed when thedifferential pressure was raised to 3.45 MPa (500 psi). On the otherhand, the stiff 280-micron nylon fibers retained their efficiency overthis pressure range throughout the tests. This means that stiff fiberplugs will be much more stable under downhole conditions, for example,if surging and/or swabbing were encountered, or simply if the effectivecirculation density (ECD) during drilling was increased.

Another series of tests was conducted with a flow loop featuring with agrid containing thirty 3-mm holes (FIG. 25). The fluids with fibers andsolids were pumped across the grid, and the filtrate exited the flowloop perpendicular to the main flow direction. The initial flow ratethrough the loop was 378 L/min (100 gal/min). In experiments duringwhich the grid was plugged, pumping was stopped and then resumed at 756L/min (200 gal/min) to determine the plug resistance to the wall shearstress imposed by the fluid flow. The wall stress is estimated to be 35Pa at the higher flow rate. The total fiber concentration for thesetests was fixed at 1.68 mL per 300 mL of the base fluid of Table 5.

Under the dynamic flow conditions, the GL fibers were able to plug the3-mm holes of the grid at a pump rate of 378 L/min. However, whenpumping was resumed at 756 L/min, the plugs quickly eroded away. For ablend of stiff 280-micron nylon fibers (TYNEX™) and 12.5-micron flexibleFM fibers (SPECTER™), the 3-mm holes were also plugged readily at 378L/min. However, when pumping was resumed at 756 L/min, there was no signof erosion, the plug was stable, and no fluid loss was observed. Thepresence of stiff fiber strengthened the plugs, and therefore increasedthe overall resistance against erosion (as shown in Table 9).

TABLE 9 Comparison of plug stability under dynamic flow conditions. Gridwith Flow Rate 2 mm Wall shear Fiber [mL] [GPM] holes stress, PaBehavior Glass [1.68] 200 Plugged 35 Plug dislodged immediately from thegrid Blend of Stiff 200 Plugged 35 Plugs were stable and Flexible andcould not [1.68 mL] be dislodged

Example 11 Resistance to Wall Shear Stress

Four experiments were performed to evaluate the ability of fibrousfilter cakes to resist erosion arising from tangential shear stress. Thefirst two experiments involved the laboratory apparatus shown in FIG.26. The equipment features a 3-cm (1.2-in.) diameter pipe 10, withanother 3-cm diameter pipe 11 fitted perpendicularly. A filter cake 12is fitted at the junction between the two pipes. Filtrate 13 iscollected at the other end of pipe 11. The apparatus also has aflowmeter 14. Water is pumped through pipe 10 subjecting the filter caketo tangential stress. The filter-cake performance is monitored byobserving the volume of filtrate collected from pipe 11.

The first experiment involved glass fibers and a water-base mud whosecomposition is given in Table 10. 20 vol % of coarse CaCO₃ particleswere present, because glass-fiber filter cakes need coarse particles tosurvive sudden pressure changes.

A filtercake containing 1.68 mL of glass fibers was transferred to theapparatus. Water was pumped through the apparatus at 30 L/min,corresponding to a shear stress of 1.53 Pa. Generally, the shearstresses experienced in the annulus may be between 15-25 Pa duringcementing and 25-30 Pa during drilling.

TABLE 10 Composition of Water-Base Mud 1 WBM-1 Water (lbm/bbl) [kg/m³](262) [749] Bentonite (lbs/bbl) [kg/m³] (21) [60] LCM 1: CaCO₃ of 10 μm(lbm/bbl) [kg/m³]  (44) [126] LCM 2: CaCO₃ of 130 μm (lbm/bbl) [kg/m³](172) [493] Density (lbm/gal) [SG] (11.8) [1.41] Solid Loading 25%

The second experiment involved a water-base mud whose composition isgiven in Table 11. The fluid also contained 2.00 mL of a blend of stiff(aspect ratio 10 to 100) and flexible (aspect ratio 200 to 1000) fibers.The stiff fiber was NL, and the flexible one was FM (SPECTER).

A 2.00-mL fiber-blend filter cake was deposited across a 3-mm opening.The fiber blend plugged the opening and sustained overburden-pressurechanges from 7 to 42 MPa (100 to 600 psi). The fiber-blend filtercakewas then transferred to the apparatus. Water was pumped through theapparatus at 30 L/min, corresponding to a shear stress of 1.53 Pa.

TABLE 11 Composition of Water-Base Mud 2 WBM-2 Bentonite (lbm/bbl)[kg/m³] (21) [60] Barite (lbm/bbl) [kg/m³]  (69) [198] LCM 1: CaCO₃ of1000 μm (lbm/bbl)  (45) [129] [kg/m³] LCM 2: CaCO₃ of 130 μm (lbm/bbl)[kg/m³] (133) [381] Density (lbm/gal) [SG] (12.5) [1.50] Solid Loading25.7%

The results of both experiments are shown in FIG. 27. The glass fiberscould not sustain the tangential erosion, and began to fail at a flowrate of 3.0 L/min. However, the filter cake containing the blend ofstiff and flexible fibers did not leak at the maximum flow rate of 30L/min.

The flexible fibers have an intrinsic tendency to pass through thefracture; however, the stiff fibers anchor them in place. Nevertheless,some of the flexible fibers manage to pass through the fracture,dragging a few stiff fibers with them. This promotes the formation of astrong filter cake with internal plugging.

The third and fourth experiments involved the same two systems describedabove; however, a pilot-scale apparatus was employed that is capable ofsimulating the downhole shear environment—35 MPa. A schematic diagram isshown in FIG. 28. The apparatus comprises a 1600-L (10 bbl) mixing tank101 fitted with a stirrer 102. A centrifugal pump 103 transports fluidfrom the mixing tank through a 102-mm (4-in.) hose 104 into a loss-zoneunit 105. The loss-zone unit is made of transparent polycarbonate,allowing one to observe the behavior of the fluid. Inside the loss-zoneunit, the fluid passes by a grid 106 comprising holes (see FIG. 25).Grids with various hole sizes are available. The outlet 107 of theloss-zone unit is connected to a 102-mm (4-in.) hose 108 that leads backinto the mixing tank. The arrangement allows continuous fluidcirculation during a test. Fluid that escapes through the grid iscollected through a filtrate outlet 109. The inlet and filtrate outletof the loss-zone unit are fitted with pressure sensors 110 and 111, aswell as Rosemount flowmeters 112 and 113. Fluid exiting the filtrateoutlet travels through a 25-mm (1-in.) hose 114 back into the mixingtank.

For each experiment, 1600 L (10 bbl) of fluid were prepared. Asdescribed in Table 12, the glass-fiber filter cake was easily removed at35 MPa shear, while the flexible-fiber blend filter cake survived.

TABLE 12 Fiber Filter Cake Performance Under Realistic Shear-StressConditions Grid Flow Rate with Wall [L/min 2-mm shear Fiber [mL](gal/min)] holes stress, Pa Behavior Glass [1.68] 760 (200) Plugged 35Plug dislodged immediately from the grid Blend of Stiff 760 (200)Plugged 35 Plugs were stable and and Flexible could not be dislodged[1.68]

Example 12 Controlling Spurt by Adjusting Ratio Between Stiff andFlexible Fibers

Four fluids with the composition described in Table 10 were prepared.Each fluid contained a different fiber design: 100 vol % stiff (NL), 100vol % glass (GL), 90 vol % stiff; 10 vol % flexible (FM Specter), and 70vol % stiff; 30 vol % flexible. The fiber concentration during each testwas 1.68 mL/300 mL. Each fluid was placed into the apparatus describedin FIG. 1, which was fitted with a 3-mm slot. The pressure was increasedfrom 0.69 MPa (100 psi) to 2.45 MPa (500 psi) in 0.69 MPa increments.The results are shown in FIG. 29.

The 100 vol % glass fiber fluid and the 100 vol % stiff fiber fluidfailed to deposit a barrier to flow. However, the fluids containingstiff and flexible fibers provided robust control. Lower spurt wasobserved with the fluid that contained 30 vol % flexible fibers.

Example 13 Filter-Cake Strength Determination

The effects of fiber compositions on the filter-cake strength werestudied. Three fluids with the composition described in Table 10 wereprepared. The first fluid contained 14.3 kg/m³ (5.0 lbm/bbl) of glassfiber (GL). The second fluid contained 6.0 kg/m³ (2.1 lbm/bbl) of amixture of stiff and flexible fibers (70 vol % stiff (NL): 30 vol %flexible (FM Specter). The third fluid contained 6.0 kg/m³ (2.1 lbm/bbl)of a mixture of stiff and flexible fibers (90 vol % stiff: 10 vol %flexible). Each fluid was placed into the apparatus described in FIG. 1,which was fitted with a 1-mm slot.

Filter cakes were prepared at 3.45 MPa (500 psi) pressure, and thentransferred to a TA-HD Plus Texture Analyzer to determine thefilter-cake strength. The Texture Analyzer is manufactured by TextureTechnologies, Scarsdale, N.Y., USA. A filter cake is placed on a grid,and the apparatus forces a blade through the filter cake. A load cellmeasures the force necessary to penetrate the filter cake.

The results are presented in FIG. 30. The 70:30 stiff:flexible filtercake was the strongest, followed by the 90:10 stiff:flexible filter cakeand the glass filter cake.

1. A method for blocking fluid flow through a pathway in a subterraneanformation penetrated by a wellbore, comprising: (i) selectingcompositions, concentrations and dimensions of stiff fibers and solidplugging particles; (ii) preparing a blocking fluid comprising the stifffibers and a blend of the solid plugging particles; and (iii) forcingthe blocking fluid into the pathway, wherein the stiff fibers have avalue of the parameter ${S = \frac{E\; d^{4}}{W\; l^{3}}},$ where S isthe stiffness, E is Young's modulus, d is the diameter, W is the forcecausing a deflection, and 1 is the length; and the value of S is about 2to about 400,000 times that of a fiber having a modulus of 65 GPa, adiameter of 20 microns, and a length of 12 mm.
 2. The method of claim 1,wherein the stiff fibers form a mesh across the pathway, and the solidparticles plug the mesh, thereby blocking fluid flow.
 3. The method ofclaim 1, wherein the stiff fibers have a Young's modulus of from about0.5 to about 100 GPa.
 4. The method of claim 1, wherein the stiff fibershave a shortest cross-sectional distance of from about 80 to about 450microns.
 5. The method of claim 1, wherein the length of the stifffibers is between about 5 to about 24 mm.
 6. The method of claim 1,wherein the stiff-fiber concentration is from about 2.85 kg/m³ to about42.8 kg/m³.
 7. The method of claim 1, wherein the fluid furthercomprises fibers that are selected from non-stiff fibers, differingstiff fibers, or both.
 8. The method of claim 7, wherein the total fiberconcentration is from about 2.85 to about 42.8 kg/m³.
 9. The method ofclaim 7, wherein the non-stiff fibers have a Young's modulus of fromabout 0.5 to about 10 GPa.
 10. The method of claim 7, wherein thenon-stiff fibers have a shortest cross-sectional distance of from about10 to about 100 microns.
 11. The method of claim 1, wherein theconcentration or composition of the stiff fibers is varied.
 12. Themethod of claim 1, wherein the blocking fluid further comprises mica.13. The method of claim 1, wherein the solid volume fraction (SVF) ofthe solid plugging particles is between 8 and 50 volume percent.
 14. Themethod of claim 1, wherein the solid plugging particles are present asfine, medium and coarse particles, wherein the average particle size ofthe fine particles is smaller than 30 microns, the average particle sizeof the medium particles is between 30 microns and 180 microns, and theaverage particle size of the coarse particles is between 180 micron and1000 microns.
 15. The method of claim 1, wherein the solid pluggingparticles comprise calcium-carbonate particles.
 16. The method of claim1, wherein the pathway has one dimension across it of at least about 1mm.
 17. The method of claim 1, wherein the pathway is a hydraulicfracture, and fluid flow into the fracture is blocked.
 18. A method fortreating lost circulation in a well in a subterranean formationpenetrated by a wellbore, having one or more pathways in the formationthrough which fluids escape the wellbore and enter the formation,comprising: (i) selecting compositions, concentrations and dimensions ofsolid plugging particles and fibers, the fibers being stiff fibers, or amixture of stiff and non-stiff fibers; (ii) preparing a blocking fluidcomprising the fibers and a blend of the solid plugging particles; and(iii) pumping the blocking fluid into the well continuously until fluidflow into the formation is satisfactorily reduced, wherein the stifffibers have a value of the parameter${S = \frac{E\; d^{4}}{W\; l^{3}}},$ where S is the stiffness, E isYoung's modulus, d is the diameter, W is the force causing a deflection,and 1 is the length; and the value of S is about 2 to about 400,000times that of a fiber having a modulus of 65 GPa, a diameter of 20microns, and a length of 12 mm.
 19. A method for treating lostcirculation in a well in a subterranean formation penetrated by awellbore, having one or more pathways in the formation through whichfluids escape the wellbore and enter the formation, comprising: (i)selecting compositions, concentrations and dimensions of solid pluggingparticles and fibers, the fibers being stiff fibers, or a mixture ofstiff and non-stiff fibers; (ii) preparing a blocking fluid comprisingthe fibers and a blend of the solid plugging particles; and (iii)placing a discrete, desired quantity of blocking fluid adjacent toand/or into a well interval where one or more pathways exist, whereinthe stiff fibers have a value of the parameter${S = \frac{E\; d^{4}}{W\; l^{3}}},$ where S is the stiffness, E isYoung's modulus, d is the diameter, W is the force causing a deflection,and 1 is the length; and the value of S is about 2 to about 400,000times that of a fiber having a modulus of 65 GPa, a diameter of 20microns, and a length of 12 mm.